Seasonal cues in tropical organisms. Rainfall? Not necessarily!

Activity seasons of tropical organisms often start, on the average, at or about the beginning of the rainy or dry seasons. The hypothesis that the onset or cessation of the wet season provides the seasonal cues necessary of the initiation of the activity season of some tropical organisms is tested with data on Panamanian cicadas. Seasonal adult activity patterns are described for cicada species in Panama, mostly from Barro Colorado Island (BCI), some from Las Cumbres. In all species the correlation between the timing of the beginning and the end of the cicada season was low and not significant, so that the actual beginning data of a cicada season in a particular year had little or no predictive value for the end date. Seven out of eleven species on BCI started their average activity season at the average beginning of the dry season (one species) or rainy season (six species). Nevertheless, in 13 years, correlations between the start or end of the cicada seasons and that of the meteorological seasons were low and not significant. At best, the beginning and end of the rains played a minor role as seasonal cues governing cicada emergence or the termination of the cicada season. It is speculated that photoperiod might be a major seasonal cue governing emergence, through its effects on the host plants.